"Glass" grips were to my best knowledge manufactured to resemble pure amber. They consist of a translucent yellow material. Some are more translucent some less. Mostly there are included small particles of any material eg minute brass plates) to resemble the inside cracs and also material within real amber.
Real amber also can be melted and reformed(pressed) at certain temperature, I am not sure if this process was also used for army grips.
The terminus "glass" grip I think came first from the translucent material and second from the extremely brittle material. It is nealy impossible to get one without chips or at least cracks.
I am not aware of a slant glass grip but this does not say that they could not exist.
Most glass grips can be found on armies, on rare occasions there are grips which do resemble glass grips mated with 2nd lufts.
Here is the pic of my perfect glass gripped Siegfried/PACK army.
Regards.

PACK army 001.jpg (24.98 KB, 158 downloads)
wotan, gd.c-b#105

"Never look for sqare eggs" as a late owner of an original FHH-dagger used to say.